1. Field of the Invention
The present invention relates to a vibration wave driving apparatus such as an ultrasonic motor which generates a vibration in an elastic member and gives a driving force to a moving member that is in contact with the elastic member by using the vibration in the elastic member, and a driving circuit therefore.
2. Related Background Art
An ultrasonic wave motor, which is one of the vibration wave driving apparatuses and which generates a travelling wave in an elastic member constructing a stator, generates a plurality of standing waves, each having a different phase, in the stator constructed of the elastic member and a piezoelectric element, and compose the standing waves to generate the traveling wave.
FIG. 8 shows an electrode pattern of a piezoelectric element disposed on an annular stator of the ultrasonic motor. An electrode region at a substantially right half side of FIG. 8 shows a first electrode group for generating a first standing wave (A-phase), and an electrode region at a substantially left side of FIG. 8 shows a second electrode group for generating a second standing wave (B-phase). When a driving signal is supplied to the first electrode group by a driving circuit (not shown), a first standing wave vibration is generated in the entire stator. When a driving signal is supplied to the second electrode group by a driving circuit (not shown), a second standing wave vibration is generated in the entire stator. The first standing wave and the second standing wave are equal in wavelength λ to each other, and the first electrode group and the second electrode group are disposed in such a manner that the phases of those first and second electrode groups are shifted from each other by ¼ of wavelength λ. When those two standing waves are generated with the temporal shift of 90 degrees, the travelling wave is generated in the elastic member.
In case of the ultrasonic motor of the above system, the first standing wave generated in the first electrode group attenuates in the vibration amplitude more as the first standing wave is far from the first electrode group, and the vibration wavelengths of the first standing wave become different from each other. Similarly, the second standing wave generated in the second electrode group attenuates in the vibration amplitude more as the second standing wave is far from the second electrode group, and the vibration wavelengths of the second standing wave become different from each other.
In other words, the vibration amplitude and the vibration wavelength of the A-phase at the first electrode group portion are not completely equal to those at the second electrode group portion, and the vibration amplitude and the vibration wavelength of the B-phase at the first electrode group portion are also not completely equal to those at the second electrode group portion. As a result, the vibration amplitudes of the travelling wave obtained by synthesizing the first standing wave and the second standing wave are caused to be different from each other depending on the portions of the elastic member, thereby lowering the output of the ultrasonic motor. Also, when the ultrasonic motor is driven for a long period of time in a state where the vibration amplitude is varied, the amount of abrasion of the vibration member is different depending on the portions of the elastic member, and the output of the ultrasonic motor is lowered with lapse of time according to the fluctuation in the amount of abrasion.
In order to solve the above-mentioned problem, Japanese Patent Application Laid-Open No. 2000-333477 discloses that an electrode group for generating a first standing wave (A-phase) and an electrode group for generating a second standing wave (B-phase) are divided in such a manner that the first and second electrode groups are alternately arranged with spaces of ¼ of a wavelength λ at the respective boundaries as shown in FIG. 9. This structure makes it possible to suppress the degree of attenuation of the vibration amplitude of the standing wave.
However, the invention disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 2000-333477 is merely a method of slightly suppressing the degree of the variation in the vibration amplitude. Consequently, a phenomenon in which the vibrations of the first standing wave and the second standing wave are not uniformly excited over the entire vibration member is not eliminated, and differences still occur in the amplitude and the wavelength of the travelling wave depending on the portions of the elastic member.
There are other factors that cause variation in the amplitude and the wavelength of the travelling wave.
Electrode patterns shown in FIG. 8 and FIG. 9 are formed in such a manner that electrode regions which are polarized in a positive direction (+) toward a thickness direction of a piezoelectric element and electrode regions which are polarized in a negative direction (−) toward the same are adjacent to each other. The lines of electric force extend in parallel with the thickness direction in the substantially center portion of the electrode regions which are polarized in the positive direction and the substantially center portion of the electrode regions which are polarized in the negative direction. However, the lines of electric force extend over the electrode regions in a direction orthogonal to the thickness direction on the boundary portions between the electrode regions which are polarized in the positive direction and the electrode regions which are polarized in the negative direction. Because a difference occurs in the longitudinal elastic coefficient of the piezoelectric element depending on the directions along which the lines of electric force extend, the rigidity of the piezoelectric element is different depending on the portions thereof. For that reason, the propagation speed of the travelling wave partially changes, and variation occurs in the vibration amplitude and the wavelength of the travelling wave.
In order to improve the variation in the rigidity which is caused by a difference in the polarizing direction as described above, Japanese Patent Application Laid-Open No. 2001-157473 proposes the structure of a piezoelectric element shown in FIG. 10. The electrode pattern of the piezoelectric element divides the electrode regions every ¼ of the vibration wavelength λ, and allocates those electrode regions alternately as the first electrode group that generates the first standing wave and the second electrode group that generates the second standing wave. The electrode regions for generating those standing waves are polarized in the same direction over the entire electrode regions. Since the adjacent electrode regions are polarized in the same direction, all of the lines of electric force in those electrode regions extend in parallel to the thickness direction, and no difference occurs in the rigidity of the piezoelectric element depending on the portions thereof.
The structure shown in FIG. 10 is directed to a mode suitable for 4-phase driving as will be described later and can be realized even in other driving modes such as 3-phase driving. In order to realize such other driving modes, there are provided electrode regions equal to integer times the standing waves which are synthesized on the electrode regions corresponding to the wavelength λ of the travelling wave, and those electrode regions are polarized in the same direction.
Returning to FIG. 10, the same driving signals are supplied to the electrode regions of +A phase which are arranged every three regions. A driving signal which is temporally shifted by 90 degrees from the driving signal of +A phase is supplied to the electrode regions of +B phase which are adjacent to the electrode regions of +A phase in the clockwise direction. A driving signal which is opposite in phase to the driving signal of +A phase is supplied to the electrode regions of −A phase which are adjacent to the electrode regions of +B phase in the clockwise direction. Then, a driving signal which is opposite in phase to the driving signal of +B phase is supplied to the electrode regions of −B phase which are adjacent to the electrode regions of −A phase in the clockwise direction. Those respective electrode regions are arranged at intervals of ¼ of the wavelength λ of the standing wave generated by supplying the driving signal thereto. When the above-mentioned driving signals are supplied to the piezoelectric element, the first standing wave and the second standing wave which are shifted in phase from each other by ¼ of the wavelength λ are generated in the piezoelectric element. Because both of the first electrode group that generates the first standing wave and the second electrode group that generates the second standing wave are arranged at regular intervals over the entire periphery of the piezoelectric element, no variation occurs in the travelling wave.
FIG. 11 is a block diagram showing the circuit structure of an ultrasonic motor using the piezoelectric element shown in FIG. 10. An oscillation circuit 1 generates an ac signal corresponding to the driving frequency of the ultrasonic motor and the ac signal is inputted to a phase shifter 2 to generate an ac signal which is shifted in phase by 90 degrees. The ac signal which is shifted in phase by 90 degrees is inputted to another phase shifter 2 to generate an ac signal which is shifted in phase from the ac signal obtained by the oscillation circuit 1 by 180 degrees. The ac signal which is shifted in phase by 180 degrees is inputted to still another phase shifter 2 to generate an ac signal which is shifted in phase from the ac signal obtained by the oscillation circuit 1 by 270 degrees. In this way, the ac signals different in phase by 90 degrees, respectively, are generated, and then boosted up to voltages that enable the ultrasonic motor to be driven by the booster circuit 3, to thereby generate +A phase voltage, −A phase voltage, +B phase voltage and −B phase voltage which are shifted in phase by 90 degrees in the stated order, respectively. Those voltage waveforms are shown in FIG. 12.
When those +A phase voltage, −A phase voltage, +B phase voltage and −B phase voltage are supplied to the above-mentioned electrode region of +A phase, electrode region of −A phase, electrode region of +B phase and electrode region of −B phase, respectively, the travelling wave which is uniform in vibration amplitude is generated. With this structure, there can be realized the ultrasonic motor whose output is high and whose stator is not partially abraded.
When the electrode pattern of the piezoelectric element is formed as shown in FIG. 10, the vibration amplitude of the travelling wave can be made uniform. However, there is required a circuit for generating the ac signal of four phases as shown in FIG. 11.
Also, in the structure shown in FIG. 10, the electrode region of a sensor phase for detecting the vibration of the vibration member cannot be ensured. This is because when the electrode region which becomes the sensor phase is provided, the driving signal is not supplied to only the electrode region which becomes the sensor phase, and a variation occurs in the vibration amplitude.